Need Help Removing a Green Screen, Movie Clip Node missing
First of all make sure you are in the compositor (not the material editor or the texture editor) and have Use Nodes enabled:



To add a Movie Clip node use ShiftA Input->Movie_Clip



Select your image and add a Keying node. Connect your image and select the color you want to use as key.



Then just plug that to an Alpha over node to mix with the 3D scene (or other image source).